1. Field of the Invention
The present invention relates to a novel method of manufacturing a vinyl chloride-coated steel sheet. More specifically, the present invention is directed to a method of manufacturing a vinyl chloride-coated steel sheet which comprises coating the surface of a thin metal sheet, as the raw material, such as a zinc-plated steel sheet, with electron ray curable adhesives, laminating a vinyl chloride film to the coated surface, and then irradiating the upper surface of the coated sheet with electron rays to thereby bonding the film to the metal sheet.
2. Description of Background and Material Information
Vinyl chloride-coated steel sheets have conventionally been prepared by coating the surface of thin metal sheets, such as zinc-plated steel sheet or cold rolled steel sheet, with vinyl chloride resin paint, followed by applying heating and baking. The vinyl chloride resin paint used in this case is commonly referred to as a vinyl resin paste (hereinafter referred to as a "paste") composed of a vinyl chloride resin normally used in paste polymerized emulsion polymerization and DOP, or other similar plasticizer, stabilizer and other ingredients admixed therewith into a viscous flowing liquid product, which is coated to a predetermined thickness on a steel sheet by a fixed blade method or reversed roll method, and heated in a heating furnace to thereby gel the coating film of the vinyl chloride resin paste.
The above-mentioned method (hereinafter referred to as a "paste coating method") involves the following drawbacks.
(1) Since a relatively large amount of plasticizer has to be used in order to obtain a flowing paste suitable to the coating process, it is difficult to obtain a coating film with low plasticizer content (the coating film of lower plasticizer content has higher film strength and is less contaminated).
(2) Since the viscosity of the paste has to be lowered using plasticizers in order to increase the fabrication speed (for example higher than 60 m/min), the drawback as described in (1) above becomes even more pronounced.
(3) Heating at high temperature (for example, higher than 180.degree. C.) is necessary gel the paste so as to obtain a firm coating film and the cost of heating is enormous.
(4) If the filtration for the paste is inadequate (or) if the paste becomes contaminated coating detects or pinholes tend to occur.
(5) Since requisite thick coating can not be obtained by one application of the paste in view of the coating film strength, the coating step has to be repeated several times each of the coating steps being followed by a heat treatment to gel the paste each time.
In addition to the previously described paste coating method, a method of laminating a vinyl chloride film prepared separately and bonded to a zinc-plated steel sheet using adhesives is also known. In this case, conventional adhesives employed are generally classified as described below:
(1) Synthetic resins of excellent bondability (for example, acryl-vinylacetate copolymer) properly dissolved in a solvent . . . solution type.
(2) Synthetic resins of excellent bondability dispersed in water . . . emulsion type.
(3) Adhesives composed of low molecular weight liquid synthetic resins as the main ingredient having a curing agent added thereto for utilizing the reaction activation upon polymerizing reaction . . . , e.g., two pack curing type.
(4) Synthetic resins melted or activated upon heating . . . , e.g., hot melt type or thermoadhesion type.
(5) Adhesive layers disposed on both sides of an adherant for bonding upon contact of them . . . , i.e., contact type.
However, like the paste coating method, these methods have various drawbacks because heating is required for evaporating the solvent or water in (1) and (2), insufficient strength is obtained until the completion of the polymerizing reaction, and at least several hours of time are necessary till the completion of the polymerizing reaction in (3), and heating or drying time, i.e., oven time is necessary in the step of disposing the adhesive layers on the surfaces of the adherent in (4) and (5).
In view of disadvantage of conventional process described above, a method of using electron ray curable adhesives has been proposed to overcome these disadvantages.
As for the method, there have been various proposals, for example, in Japanese Patent Application Laid Open Nos. Sho 58-174475, 59-86676, 60-168644 etc. In the case of Japanese Patent Application Laid Open No. Sho 58-174475, a particular mixture comprising (A) from 10 to 69.95 parts by weight of a urethane polyacrylate having an average molecular weight of 500 to 20,000, (B) from 10 to 40 parts by weight of a polyol polyacrylate, (C) from 20 to 79.95 parts by weight of a radical polymerizable monovinyl compound and (D) from 0.05 to 10 parts by weight of a radical polymerizable phosphor-containing compound, as an electron ray curable adhesive, is coated onto a steel sheet to a film thickness of about 6 .mu.m, and a soft vinyl chloride sheet of 0.2 mm thickness is laminated, as a vinyl chloride coating layer, to the upper surface thereof followed by irradiation with electron rays at an acceleration voltage of 300 KV and at a dose of 20 Mrad. While the time for the irradiation is dependent on the dose, the adhesives are usually cured on the order of several seconds, thereby obtaining a vinyl chloride-coated steel sheet having an intense bonding strength.
The electron ray curing method of the present invention is different from the usual adhesion method, as described above, since the substances used as adhesives have a 100% solid content, i.e., no solvent, and do not require a separate drying in as much as the electron ray curing method requires only electron ray irradiation as the means for causing the adhesive layer to exhibit effective bonding force with the adhesives this method eliminates any need for heating otherwise required for this purpose.
Consequently, this results in various advantages in the method of manufacturing a vinyl chloride-coated steel sheet. First, inasmuch as neither drying nor heating means are required in the bonding step and the requisite bonding cure can be obtained merely by irradiation with electron rays as described above there by simplifying the manufacturing process and enhancing high speed production. Further, since heating is not necessary either for drying, gelation or curing of the adhesives during the manufacturing step for the vinyl chloride-coated steel sheet, the decorative appearance of the laminated soft vinyl chloride sheet can be as it is maintained, if desired. Related to this, in the case of a conventional vinyl chloride-coated steel sheet, an embossed design is typically applied to the surface of the vinyl chloride coating by passing the vinyl chloride-coated steel sheet through an embossing device. However, since the vinyl chloride coating layer is thin, only the thin and shallow embossed design, such as a feather-or satin-like embossed pattern can be obtained, which is often unsatisfactory from the aesthetic or decorative point of view. In the case of the bonding method, however, since an optional embossed design can be applied to the surface of a previously molded soft vinyl chloride sheet, it is possible to use those applied with hessian design, leather-like design in addition to various types of reflective embossed designs. However, in the case of a conventional bonding method, as described above, since the production step inevitably involves a heating step, subjecting the soft vinyl chloride sheet applied with such various designs to heating often causes a fatal defects such that the embossed design applied to the surface is eliminated or offset by the heating. Inasmuch as such a heating is not required in the case of the electron ray curing method in this regard, however, various embossed designs which may be applied to the surface of the soft vinyl chloride sheet are not degraded and the shape can be maintained as it is, and, accordingly, a vinyl chloride-coated steel sheet exhibiting excellent decorative characteristics that can not be obtained in a conventional vinyl chloride-coated steel sheet can be obtained by using the electron ray curing method of the present invention.
Since the electron ray curing method of the present invention is a novel manufacturing method for the vinyl chloride-coated steel sheet having such an advantageous feature, it would be expected that the electron ray curing method will become a popular substitute for the conventional paste coating method and extensive studies have now been made in attempts to discover suitable electron ray curable adhesives. However, it has been found that there are difficulties in the method of manufacturing the vinyl chloride-coated steel sheet by the electron ray curing method, which hinders the industrial use of the method. One of the main obstacles to overcome is that the vinyl chloride sheet laminated on the surface of the steel sheet with an aim of decorative coating and protective coating is remarkably degraded by the irradiation dose of electron rays required to cure the adhesives. A dose higher than a certain level is required for curing the electron ray curable adhesives and a vinyl chloride sheet is not degraded if the dose is lower. In this regard, it has been found that a dose of higher than 10 Mrad is required to cure the adhesives and all of the conventional vinyl chloride sheets are degraded by the electron rays at such a dosage, although this is not the case if the irradiation dosage is lower.
Vinyl chloride-coated steel sheets have various uses, such as roofing materials, exterior materials, decks and fences for verandas, portable warehouse, and similar outdoor applications. Accordingly, the vinyl chloride sheet used in the method is unavoidably subjected to UV-ray irradiation after undergoing the irradiation of electron rays at a predetermined dose and is exposed under extremely severe degrading conditions that can not be recognized at all in the degradation test known so far.